1. Field of the Invention
The present invention relates to an apparatus and method for use in a wellbore. More particularly, the invention relates to a downhole tool for determining the location of an obstruction in a wellbore. More particularly still, the invention relates to a downhole tool for locating the point at which a tubular such as a drill string is stuck in an opening or a hole such as a hollow tubular or a wellbore.
2. Description of the Related Art
As wellbores are formed, various tubular strings are inserted into and removed from the wellbore. For example, a drill bit and drill string are utilized to form the wellbore which will typically be lined with casing as the bore hole increases in depth. With today's wells, it is not unusual for a wellbore to be several thousand feet deep with the upper portion of the wellbore lined with casing and the lowest portion still open to the earth. As the well is drilled to new depths, the drill string becomes increasingly longer. Because the wells are often non-vertical or diverted, a somewhat tortured path can be formed leading to the bottom of the wellbore where drilling takes place. Because of the non-linear path through the wellbore, the drill string can become bound or other wise stuck in the wellbore as it moves axially or rotationally. The issues related to a stuck drill string are obvious. All drilling operations must be stopped and valuable rig time lost. Because the drill string is so long, determining the exact location of the obstruction can be difficult.
Because of the length of the drill string and the difficulty in releasing a stuck drill string it is useful to know the point at which one tubular is stuck within another tubular or within a wellbore. Such knowledge makes it possible to accurately locate tools or other items above, adjacent, or below the point at which the tubular is stuck. The prior art includes a variety of apparatuses and methods for ascertaining the point at which a tubular is stuck.
The most common apparatuses and methods employ the principle that the length of the tubular will increase linearly when a tensile force is applied, so long as the tensile force applied is within a given range. The range of linear response is based on many factors, including the mechanical properties of the tubular such as the yield strength of the material. One method of determining an approximate location for the sticking point of a tubular involves applying a known tensile force to the tubular and measuring the elongation at the surface of the well. If the total length of the tubular within the second tubular or wellbore is known, then the total amount of theoretical elongation can be calculated, based on the assumption that the applied force is acting on the entire length of the tubular. Comparing the measured elongation to the theoretical elongation, one can estimate the sticking point of the tubular. If the measured elongation is fifty percent of the theoretical elongation, then it is estimated that the tubular is stuck at a point that is approximately one half of the length of the tubular from the surface. Several factors have a negative impact on the accuracy of this method. Among these are the friction between the tubular and the surface in which it is stuck and the changes in the properties of the tubular due to corrosion or other conditions.
This same principle of applying a known force to the stuck tubular and measuring the response can also be used to more accurately determine the location of the sticking point. By placing a freepoint tool at the end of a run in string within the stuck tubular, one can accurately determine the sticking point location by placing the tool at various locations within the tubular, applying a known tensile force, and accurately measuring the elongation of the tubular at the location of the freepoint tool. A similar method utilizes a known torque applied to the tubular and measurement of the rotational displacement. In both methods, a freepoint tool is placed at a location within the tubular and then anchored to the tubular at each end of the freepoint tool. If the portion of the pipe between the anchored ends of the freepoint tool is elongated when a tensile force is applied (or twisted when a torsional force is applied) at the surface to the stuck tubular, it is known that at least a portion of the freepoint tool is above the sticking point. If the freepoint tool does not record any elongation when a tensile force is applied (or twisting when a torsional force is applied) at the surface to the stuck tubular, it is known that the freepoint tool is completely below the sticking point. By moving the freepoint tool within the stuck tubular and measuring the response in different locations to a force applied at the surface, the location of the sticking point may be accurately determined.
A common problem associated with freepoint tools is the need to provide both a means of positively anchoring the ends of the freepoint tool when a measurement is being taken and also being able to freely move the tool to a new location within the tubular. A common type of anchoring system utilizes a bow spring to anchor the freepoint tool to the inside surface of the stuck tubular. A problem associated with this system is that the bow springs are in constant contact with the inside surface of the stuck tubular as the freepoint tool is being lowered into the stuck tubular on a run in string. It is difficult to set the bow springs so that there is enough friction between the spring and the stuck tubular to allow for accurate measurement of the response to a force on the stuck tubular, yet permit the freepoint tool to be moved from one location to another.
Another method of anchoring a freepoint tool to a stuck tubular utilizes motorized “dog type” anchors. With these systems, a motor is typically used in conjunction with a gear system or other mechanical arrangement to actuate the anchors and drive them into the wall of the stuck tubular. To ensure positive engagement of the anchoring system, the motor is typically driven until it is stalled by the wall of the stuck tubular restricting movement of the anchor. This technique can lead to overheating of the motor and eventual failure of the motor windings. Another problem associated with this type of arrangement occurs when attempting to anchor the freepoint tool in a horizontal section of a stuck tubular. In this situation, the anchor must lift up the freepoint tool from the bottom of the stuck tubular to fully engage anchors. The weight of the freepoint tool may stall the motor before the anchor system is fully engaged and therefore prevent a measurement of the response of the tubular.
In addition, protecting the freepoint tool sensors that detect the response of the tubular from the harsh environment of a wellbore is another problem. The sensors utilized are typically fragile components that can not operate in the extreme pressures and temperatures often found in a wellbore. Typical freepoint tool designs utilize an oil-filled chamber in combination with a piston to hydrostatically balance them with the wellbore pressure, but this complicates the assembly and repair of the freepoint tool and disturbs measurements at high temperatures.
Another problem associated with freepoint tools is the need to generate large forces acting on the tubular at the surface in order to generate a response that is capable of being detected by the sensors of the freepoint tool. This problem is exacerbated by sensors that do not have sufficient sensitivity or accuracy. An additional problem exists in the need to accurately and quickly reset the freepoint tool after a measurement has been taken so that a new measurement may be taken in a different location within the tubular. It is necessary to quickly reset the freepoint tool in situations where measurements will be taken in several different locations. It is also necessary to reset the freepoint tool in an extremely accurate manner due to the small magnitude of the responses that will be measured by the freepoint tool.
A need therefore exists to provide a more accurate means for locating a point where a tubular is stuck in a wellbore. There is a further need for both a means to positively anchor the ends of a freepoint tool when a measurement is being taken and to freely move the tool to a new location within the tubular apparatus for new measurement locations. A further need exists for a means of protecting the freepoint tool sensors that detect the response of the tubular from the harsh environment of a wellbore. Still a further need exists for a freepoint tool that does not require the generation of large forces acting on the stuck tubular in order to generate a response that is capable of being detected by the sensors of the freepoint tool. Yet another need exists for a freepoint tool that may be accurately and quickly reset before measurements are taken to determine the response.